1. Technical Field
The present invention relates to a liquid crystal display panel. More particularly the present invention relates to a liquid crystal display panel having an ornamental reflective part (border region) for improving the appearance around the periphery of the display region, wherein dark lines do not appear alongside the scan line wiring in the border region.
2. Related Art
Over recent years the application of liquid crystal display panels has spread rapidly, not only in information and telecommunications equipment but in electrical equipment in general. Since liquid crystal display panels do not themselves emit light, the transmissive type of liquid crystal display panel that is equipped with a backlight is much employed.
However, since backlights consume large amounts of power, the reflective type of liquid crystal display panel which does not need a backlight have been used, especially for portable equipment, in order to reduce power consumption. But reflective liquid crystal display panels use external light as light source and therefore are difficult to view in dark interiors of rooms, etc. Accordingly, in recent times particular progress has been made with the development of liquid crystal display panels of a semi-transmissive type, which possess the capabilities of both the transmissive and reflective types.
Having, in each pixel region, a transmissive part equipped with a pixel electrode and a reflective part equipped with both a pixel electrode and a reflection electrode, in dark places semi-transmissive liquid crystal display panels display images by lighting a backlight and utilizing the transmissive part of the pixel region, and in bright places by utilizing external light via the reflective part, without lighting the backlight. Thus, such panels have the advantages of not needing to light the backlight always, and of being able to drastically reduce power consumption.
A specific example of a related-art semi-transmissive liquid crystal display panel is described below using FIGS. 7 to 9. FIG. 7 is a schematic plan view of a related-art single-terminal type semi-transmissive liquid crystal display panel, FIG. 8 is a plan view of one pixel portion of the array substrate in FIG. 7, and FIG. 9 is a cross-sectional view along C-C in FIG. 8. In FIG. 7, the non-display region around the periphery of the display region is depicted in an exaggerated manner for the sake of comprehension of the invention. Also, as used herein the term “display region” refers to the planar region where the pixel electrode is formed and where the alignment of the liquid crystal layer (the liquid crystal molecules) is controlled via the electric field applied to the pixel electrode, while the term “non-display region” refers to the planar region where no pixel electrode is formed and where the alignment of the liquid crystal layer, if present, is not controlled.
A related-art semi-transmissive liquid crystal display panel 10A has an array substrate 11 and a opposed substrate 12 that are opposed to each other and hold a liquid crystal layer between them. On the array substrate 11 there are formed, in the display region 14 on a transparent substrate 13 of glass or the like, parallel and equally-spaced multiple scan lines 17 constituted of aluminum, molybdenum or similar metal. Moreover, the scan lines 17 are coupled, via scan line wiring 171, to a driver circuit placement portion 16 that is provided in the frame region 15 at the periphery of the display region 14. Further, auxiliary capacitance line 18 is formed roughly centrally between adjacent scan lines 17 so as to be parallel to the scan lines 17, and in addition, a gate electrode G for TFTs is drawn out from the scan lines 17. Further, common wiring is provided on the transparent substrate 13, but is omitted from the drawings.
Also, a gate insulator 19 constituted of silicon nitride, silicon oxide or the like is laid over the entire surface of the transparent substrate 13 so as to cover the scan lines 17, auxiliary capacitance line 18 and gate electrode G, and a semiconductor layer 20 constituted of amorphous silicon, polycrystalline silicon or the like is laid over the gate electrode G, with the gate insulator 19 interposed. Further, a plurality of signal lines 21 constituted of a metal such as aluminum or molybdenum are formed over the gate insulator 19 so as to be orthogonal to the scan lines 17, and the plurality of signal lines 21 are likewise connected, via signal line wiring 211, to the driver circuit placement portion 16. Also, a source electrode S for TFTs is drawn out from these signal lines 21 so as to contact with the semiconductor layer 20, and furthermore, a drain electrode D of the same material as the signal lines 21 and source electrode S are provided on the gate insulator 19, likewise so as to contact with the semiconductor layer 20.
Each region enclosed by the two adjacent scan lines 17 and two adjacent signal lines 21 is equivalent to 1 pixel. TFTs that serve as switching elements are constituted by the gate electrode G, gate insulator 19, semiconductor layer 20, source electrode S and drain electrode D, one TFT being formed for each pixel. The auxiliary capacitance of each pixel is formed by the drain electrode D and auxiliary capacitance line 18.
A protective insulator (also termed passivation film) 22 of for example an inorganic insulating material is deposited over the whole surface of the transparent substrate 13 so as to cover the signal lines 21, TFTs and gate insulator 19. Over this protective insulator 22, an interlayer (also termed a flattening film) 23 constituted of organic insulator is deposited so as to extend over the entire transparent substrate 13. A contact hole 24 is formed in the protective insulator 22 and the interlayer 23 in position corresponding to the drain electrode D of the TFTs. Further, in each pixel a reflector 27 partially constituted of aluminum or similar metal is formed in the TFT and auxiliary capacitance line 18 side, and a pixel electrode 26 constituted of for example ITO (indium tin oxide) or IZO (indium zinc oxide) is formed on the surfaces of the reflector 27, contact hole 24 and interlayer 23. An alignment layer (not shown) is deposited over the surface of the pixel electrode 26 in such a manner as to cover all of the pixels.
The opposed substrate 12 is another transparent substrate 28 constituted of glass plate or the like, for example, on the surface of which a color filter layer 29 composed of red (R), green (G) and blue (B), corresponding to individual pixels, is provided at least in a position corresponding to the display region 14 of the array substrate 11. A top coat layer 30 is deposited on the surface of this color filter layer 29 in at least the position corresponding to the region where the reflector 27 is provided, in other words to the reflective part, of the array substrate 11, and furthermore, a common electrode 31 and an alignment layer (not shown) are deposited on the surface of the top coat layer 30 and color filter layer 29. The top coat layer 30 is provided in order to cause the distance (the cell gap) between the pixel electrode 26 and the common electrode 31 at the reflective part to be approximately ½ the cell gap at the transmissive part where no reflector 27 is provided, and in order that the color tone at the reflective part and at the transmissive part will be equivalent. The color filter layer 29 may, where appropriate, be used in combination with a color filter layer of cyan (C), magenta (M) and yellow (Y), etc., while in the case of a monochrome display, it may be that no color filter layer is provided.
The semi-transmissive liquid crystal display panel 10A is then obtained by: positioning opposite each other the array substrate 11 and opposed substrate 12 obtained in the foregoing manner; deploying, at appropriate intervals around the edge portions, columnar spacers 39 for maintaining a constant cell gap; sealing with sealing agent 35 the peripheries of the array substrate 11 and opposed substrate 12; electrically coupling the common wiring of the array substrate 11 and the common electrode of the opposed substrate 12 via a transfer electrode (not shown); injecting liquid crystal into the space between the two substrates through a liquid crystal injection hole (not shown); and sealing the liquid crystal injection hole.
In such semi-transmissive liquid crystal display panel 10A, a backlight (not shown) is deployed on the array substrate 11 side; in dark places the backlight is lighted and the requisite images are displayed by means of light transmitted through the semi-transmissive liquid crystal display panel 10A, while in bright places the requisite images are displayed by utilizing reflected external light, without lighting the backlight. But if the reflector is provided over the entire rear surface of each pixel electrode 26, a reflective liquid crystal display panel will be obtained. In such a semi-transmissive liquid crystal display panel or reflective liquid crystal display panel, the reflector 27 is in some cases provided on the surface of the pixel electrode 26, and it is common practice, for the sake of achieving good reflection efficiency at the reflective part and also of producing satisfactory white displays, to provide concavoconvexities on the surface of the interlayer 23 at the places where the reflector is provided, with the purpose of making the reflected light into diffuse reflected light.
In a related-art semi-transmissive liquid crystal display panel or reflective liquid crystal display panel such as described above, the non-display region around the periphery of the display region is covered over by a light-blocking black matrix and an outer cover, so that essentially the display region alone will be visible to viewers. For example, in the related-art semi-transmissive liquid crystal display panel 10A shown in FIG. 7, the non-display region 33 has at least the hatched portions covered over by a black matrix and the outer cover, so that the display portion 14 alone is visible to the viewer.
In recent years however, there have come into use liquid crystal display panels in which, in order to improve the appearance, a reflective part that reflects external light is formed in the non-display region around the periphery of the display region, and such non-display region with reflective part formed therein is used for ornament. In a semi-transmissive liquid crystal display panel 10B that uses for ornament such non-display region with reflective part formed therein, the non-display area 33 is covered by the black matrix and outer cover and is invisible to the viewer, whereas the portion of the non-display region 34 where the reflective part is formed is visible to the viewer, as shown in FIG. 10. FIG. 10 is a schematic plan view of a semi-transmissive liquid crystal display panel 10B that uses for ornament a non-display region with a reflective part formed therein; here too, the non-display region around the periphery of the display area is depicted in an exaggerated manner. Structural elements in FIG. 10 that are the same as those of semi-transmissive liquid crystal display panel 10A in FIG. 7 are assigned identical reference numerals, and detailed descriptions thereof are omitted.
At this portion of the non-display region 34 where the reflective part is formed, no pixel electrode is provided and therefore the alignment of the liquid crystal molecules does not vary. Hence such portion is seen by the viewer as being the same color as the color filter layer, provided on the opposed substrate 12, corresponding to such portion. Usually, the color filter layer provided for such portion will be of the same kind as that formed in the display region 14, so that such portion will effectively appear white in color. To have such portion of the non-display region 34 where the reflective part is formed appear in a satisfactory white color, it is necessary, as in a reflective or semi-transmissive liquid crystal display panel, to employ almost the same reflective display structure as in the reflective part of the display region. To that end, concavoconvexities are provided on the surface of the interlayer that underlies the reflectors. Below, such ornamental portion of the non-display region 34 where the reflective part is formed is termed the “border region” and is assigned the same reference numeral “34” when described.
JP-A-2003-228049 discloses a reflective or semi-transmissive liquid crystal display panel in which the concavoconvexities provided on the surface of the interlayer at the reflective part of the display region are also provided in the non-display region, with the purpose of lessening display irregularities due to occurrence of unevenness in the cell gap near the boundary between the display region and non-display region. However, no mention is made therein of making part of the non-display region into a border region such as described above.
In a semi-transmissive liquid crystal display panel 10B having a border region 34 such as described above, the border region 34, while undergoing no change in display status, nevertheless is able to exert an aesthetic ornamental effect whereby the periphery of the display region 14 appears white at all times, so that the appearance is greatly enhanced. However, detailed investigation by the present inventors revealed that in a semi-transmissive liquid crystal display panel 10B having a border region 34 such as described above, dark lines appear alongside the scan line wiring 171 in the areas X enclosed by dashed lines on either side of the display region 14 in FIG. 10.
Upon conducting a series of various investigations into the causes of the occurrence of the night vision phenomenon alongside the scan line wiring 171 of the border region in such a liquid crystal display panel having a border region 34, the present inventors discovered that it was due to causes described below. FIG. 11 is a cross-sectional view of the frame region 15 along line D-D in the semi-transmissive liquid crystal display panel 10B of FIG. 10. In this frame region 15, plural scan line wirings 171 and common wirings 40 are formed on the surface of the transparent substrate 13 on the array substrate 11 side, and the scan line wirings 171 and common wirings 40 are covered by a gate insulator 19 and protective insulator 22. Further, in the border region 34 the surface of the protective insulator 22 is covered by an interlayer 23, and columnar spacers 39 for keeping the cell gap constant are deployed at appropriate intervals around the edge portions. Also, the peripheral portions of the array substrate 11 and opposed substrate 12 are sealed with sealing agent 35.
The region on the opposed substrate 12 where the black matrix 36 is provided forms the non-display region 33, and the area between the non-display region 33 and the display region 14 forms the border region 34. The surface of the interlayer 23 of the border region 34 is formed to have concavoconvexities, and on such concavoconvex surface of the interlayer 23 is formed a reflector 37 constituted of for example aluminum metal; furthermore, a transparent electrode 38 constituted of ITO or IZO is formed on the surface thereof, and the surfaces of both the reflector 37 and the transparent electrode 38 are formed to be concavoconvex. For the sake of balance with the process for producing the dummy electrode for static protection in the related art, the reflector 37 and the transparent electrode 38 are, as shown in FIG. 12, deposited with the same pitch as the reflector 37 and pixel electrode 26 of the display region 14, in an isolated condition such that the reflector 37 and the transparent electrode 38 are not electrically coupled to anything and are in a floating state. A black matrix is formed on the opposed substrate 12 in such a manner as to block light at the positions corresponding to the peripheries of each pixel electrode 26 of the display region 14 and of each transparent electrode 38 in the border region 34 of the array substrate 11, but is omitted in FIG. 11. FIG. 12 is an enlarged schematic view of the top left portion of the array substrate in the liquid crystal display panel 10B of FIG. 10.
Thus, it might be supposed that the portions of the border region 34 alongside the scan line wiring 171 ought not to appear dark, because the liquid crystal molecules present between the transparent electrode 38 and the common electrode 31 do not move, since there is no electric potential difference between the transparent electrode 38 and the common electrode 31, as no potential is generated in the transparent electrode 38, which is in a floating state. Yet, since the voltage applied to the scan line wiring 171 is high AC voltage (for example ±15V), the voltage applied between the scan line wiring 171 and the common electrode 31 is divided and voltage is generated in the transparent electrode 38, so that a voltage VLC is applied between the transparent electrode 38 and the common electrode 31, and due to such voltage VLC the alignment of the liquid crystal molecules between the transparent electrode 38 and the common electrode 31 varies, with the result that the phenomenon of dark appearance occurs alongside the scan line wiring 171 in the border region 34. The voltage that is applied to the signal line wiring 211 is far lower than that applied to the scan line wiring 171 and therefore effectively does not exert any influence on the liquid crystal molecules in the border region 34.
The voltage VLC that occurs between the transparent electrode 38 and the common electrode 31 will now be described using FIG. 13. The average thickness L1 of the interlayer 23 between the scan line wiring 171 and the transparent electrode 38 is approximately 1.45 μm, and the permittivity ε of the polyimide normally used for the interlayer 23 is 3.4, so that a capacitor CS with the interlayer 23 as dielectric body arises between the scan line wiring 171 and the transparent electrode 38. Further, the average distance L2 between the transparent electrode 38 and the common electrode 31 is approximately 2.0 μm, and the permittivity Δε of the liquid crystal layer generally used is approximately 7, so that a capacitor CLC with the liquid crystal layer as dielectric body arises between the transparent electrode 38 and the common electrode 31.
This means that the voltage V0 that is applied between the scan line wiring 171 and the common electrode 31 is divided by the series circuits of the capacitor CS that arises between the scan line wiring 171 and the transparent electrode 38, and of the capacitor CLC that arises between the transparent electrode 38 and the common electrode 31, so that the voltage VLC expressed by equation (1) below is applied between the transparent electrode 3 and the common electrode 31. As an example, where the voltage V0 applied between the scan line wiring 171 and the common electrode 31 is 15V, VLC will be approximately 6 V.
Equation 1
                                                                        V                LC                            =                            ⁢                                                Cs                                      Cs                    +                                          C                      LC                                                                      ⁢                                  V                  o                                                                                                                        =                .                .                            ⁢                            ⁢                              6                ⁡                                  [                  V                  ]                                                                                        (        1        )            
The inventors arrived at the present invention upon discovering, as a result of various investigations aimed at resolving the night vision phenomenon alongside the scan line wiring 171 of the border region 34 due to the foregoing causes, that if the transparent electrode 38 and common electrode 31 are electrically coupled and made to have equal potential, the potential difference V0 between the transparent electrode 38 and common electrode 31 can be reduced to 0 V, and thereby the night vision phenomenon alongside the scan line wiring 171 of the boarder region 34 can be eliminated.